Instrumenting callbacks of HttpNetworkTransaction to locate the source of jankiness

Instrumenting most popular callbacks of HttpNetworkTransaction to locate the source of jankiness.
Previous instrumentations showed that "DoCallback(rv);" call inside HttpNetworkTransaction::OnIOComplete
alone is responsible for 18.2 janks per hour in IO thread. I need to instrument
the code inside it to find out which part causes jank.

This is a mechanical change that adds instrumentation required to locate the
source of jankiness (i.e. a long-running fragment of code executed as a part of
the task that causes jank) in the code. See the bug for details on what kind of
jank we are after.
A number of similar CLs were landed, and none of them caused issues. They've
helped to find and fix janky code. The code of the instrumentation is highly
optimized and is not expected to affect performance. The code simply creates a
diagnostic task which is identical to ones created by PostTask or IPC message
handlers. The task gets created only in developer build and in Canary channel.

BUG=424359
Committed: https://crrev.com/29494d86acbce67d901844d75e4ba76ceb84cf4b
Cr-Commit-Position: refs/heads/master@{#302650}

[vadimt, 2014-10-30 21:07:06]

vadimt@chromium.org changed reviewers:
+ asanka@chromium.org, kaznacheev@chromium.org

[vadimt, 2014-10-30 21:07:07]

asanka@chromium.org: Please provide OWNER's approval for net/

kaznacheev@chromium.org:  Please provide OWNER's approval for devtools

[Vladislav Kaznacheev, 2014-10-31 00:40:23]

On 2014/10/30 21:07:07, vadimt wrote:
> mailto:asanka@chromium.org: Please provide OWNER's approval for net/
> 
> mailto:kaznacheev@chromium.org:  Please provide OWNER's approval for devtools

I do not feel confident enough to give OWNER's approval. I moved off the
DevTools team and I do not really know this part of the codebase. I think you
should ask vsevik@ or pfeldman@.

[vadimt, 2014-10-31 01:21:24]

vadimt@chromium.org changed reviewers:
+ pfeldman@chromium.org

[vadimt, 2014-10-31 01:22:02]

vadimt@chromium.org changed reviewers:
- kaznacheev@chromium.org

[vadimt, 2014-10-31 01:24:21]

pfeldman@, please provide OWNERs approval for devtools/ code.

kaznacheev@, you might want to consider removing yourself from OWNERs

[pfeldman, 2014-10-31 06:17:33]

lgtm

[asanka, 2014-11-04 03:06:57]

lgtm

[vadimt, 2014-11-04 15:16:52]

The CQ bit was checked by vadimt@chromium.org

[commit-bot: I haz the power, 2014-11-04 15:17:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/670993003/1

[commit-bot: I haz the power, 2014-11-04 17:04:11]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-11-04 17:04:13]

Try jobs failed on following builders:
  android_dbg_tests_recipe on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_dbg_tes...)

[vadimt, 2014-11-04 19:13:11]

The CQ bit was checked by vadimt@chromium.org

[commit-bot: I haz the power, 2014-11-04 19:14:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/670993003/1

[commit-bot: I haz the power, 2014-11-04 20:06:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-04 20:07:32]

Patchset 1 (id:??) landed as
https://crrev.com/29494d86acbce67d901844d75e4ba76ceb84cf4b
Cr-Commit-Position: refs/heads/master@{#302650}